Not displaying a Sign Out option in header after Sign in and Sign up, please check out the given code

The given code is:

<div class="header">
      <ul>
     <li><a href="#">MyLogo</a></li>
     <li><a href="index.php">Home</a></li>
     <?php if($_SESSION['user_id'] == true){
       ?>

       <li><a href="signup.php">Sign Up</a></li>
      <li><a href="signin.php">Sign In</a></li>
    <?php } else{ ?>
      <li><a href="signout.php">Sign Out</a></li>
     <?php } ?>
     <li><a href="#">Checkout</a></li>
   </ul>

 </div>

The above given code works for header section in which the Sign out option is not displaying. Please give me a logic to change this code and avail the Signout option.